Bacterial spores owe their incredible resistance capacities to molecular structures that protect the cell content from external aggressions. Among the determinants of resistance are the quaternary structure of the chromosome and an extracellular shell made of proteinaceous layers (the coat), the assembly of which remains poorly understood. Here, in situ cryo-electron tomography on lamellae generated by cryo-focused ion beam micromachining provides insights into the ultrastructural organization of Bacillus subtilis sporangia. The reconstructed tomograms reveal that early during sporulation, the chromosome in the forespore adopts a toroidal structure harboring 5.5-nm thick fibers. At the same stage, coat proteins at the surface of the forespore form a stack of amorphous or structured layers with distinct electron density, dimensions and organization. By analyzing mutant strains using cryo-electron tomography and transmission electron microscopy on resin sections, we distinguish seven nascent coat regions with different molecular properties, and propose a model for the contribution of coat morphogenetic proteins.

CEITEC Central European Institute of Technology Masaryk University 62500 Brno Czech Republic

School of Life Sciences University of Warwick Coventry UK

Univ Grenoble Alpes CNRS CEA EMBL ISBG F 38000 Grenoble France

Univ Grenoble Alpes CNRS CEA IBS F 38000 Grenoble France

University Grenoble Alpes CEA IRIG MEM F 38054 Grenoble France

University of Technology Sydney 2007 Ultimo NSW Australia

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